The sample size required was based on the assumption that the estimated success rate would be 70% in the 3000 impulses at 60 impulses per minute group and 50% in the 4000 impulses at 90 impulses per minute group, with a type I error of 0.05 and a type II error of 0.05 (power of 80%). This yielded a projected sample size of 93 stones in each group.